Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

Serotonin controls the maturation of the GABA phenotype in the ventral spinal cord via 5-HT1b receptors

Serotonin (5-hydroxytryptamine or 5-HT) is a pleiotropic neurotransmitter known to play a crucial modulating role during the construction of brain circuits. Descending bulbo-spinal 5-HT fibers, coming from the caudal medullary cell groups of the raphe nuclei, progressively invade the mouse spinal cord and arrive at lumbar segments at E15.5 when the number of ventral GABA immunoreactive (GABA-ir) interneurons reaches its maximum. We thus raised the question of a possible interaction between these two neurotransmitter systems and investigated the effect of 5-HT descending inputs on the maturation of the GABA phenotype in ventral spinal interneurons. Using a quantitative anatomical study performed on acute and cultured embryonic mouse spinal cord, we found that the GABAergic neuronal population matured according to a similar rostro-caudal gradient both in utero and in organotypic culture. We showed that 5-HT delayed the maturation of the GABA phenotype in lumbar but not brachial interneurons. Using pharmacological treatments and mice lacking 5-HT(1B) or 5-HT(1A), we demonstrated that the 5-HT repressing effect on the GABAergic phenotype was specifically attributed to 5-HT(1B) receptors.

In vitro sacral cord preparation and motoneuron recording from adult mice

We report the development of an intracellular recording technique for adult mouse motoneurons in sacral spinal cord. Based on a similar preparation for adult rat, we modified the cord preparation solution and filled the sharp electrode with a solution that has physiological osmolarity and pH. The viability of the preparation was examined by recording root reflexes. Short-latency reflexes mediated through monosynaptic transmission between S1 and S3 ventral root were reliably produced by dorsal root electrical stimuli and were stably recorded for more than eight hours. Long-lasting potentiation of the root reflex was observed by bath application of methoxamine, a noradrenergic alpha1 receptor agonist. Bath application of strychnine and picrotoxin, antagonists for glycine and GABA(A) receptors respectively, unmasked long-lasting reflexes that may contain polysynaptic components. In addition, on the background of strychnine and picrotoxin, adding methoxamine induced spontaneous ventral root activity. For intracellular recording, the motoneurons could be reliably penetrated and held for up to 30 min. In all 16 motoneurons recorded, resting membrane potential, input resistance, action potentials and repetitive firing were comparable to those of rat motoneurons. Thus, this preparation is viable and provides a new method for combined electrophysiological and genetic studies of the adult mouse spinal cord.

Serotonin 5-HT2 receptors induce a long-lasting facilitation of spinal reflexes independent of ionotropic receptor activity

Dorsal root-evoked stimulation of sensory afferents in the hemisected in vitro rat spinal cord produces reflex output, recorded on the ventral roots. Transient spinal 5-HT(2C) receptor activation induces a long-lasting facilitation of these reflexes (LLFR) by largely unknown mechanisms. Two Sprague-Dawley substrains were used to characterize network properties involved in this serotonin (5-HT) receptor-mediated reflex plasticity. Serotonin more easily produced LLFR in one substrain and a long-lasting depression of reflexes (LLDR) in the other. Interestingly, LLFR and LLDR were bidirectionally interconvertible using 5-HT(2A/2C) and 5-HT(1A) receptor agonists, respectively, regardless of substrain. LLFR was predominantly Abeta afferent fiber mediated, consistent with prominent 5-HT(2C) receptor expression in the Abeta fiber projection territories (deeper spinal laminae). Reflex facilitation involved an unmasking of polysynaptic pathways and an increased receptive field size. LLFR emerged even when reflexes were evoked three to five times/h, indicating an activity independent induction. Both the NMDA and AMPA/kainate receptor-mediated components of the reflex could be facilitated, and facilitation was dependent on 5-HT receptor activation alone, not on coincident reflex activation in the presence of 5-HT. Selective blockade of GABA(A) and/or glycine receptors also did not prevent reflex amplification and so are not required for LLFR. Indeed, a more robust response was seen after blockade of spinal inhibition, indicating that inhibitory processes serve to limit reflex amplification. Overall we demonstrate that the serotonergic system has the capacity to induce long-lasting bidirectional changes in reflex strength in a manner that is nonassociative and independent of evoked activity or activation of ionotropic excitatory and inhibitory receptors.

Rat embryonic motoneurons in long-term co-culture with Schwann cells—a system to investigate motoneuron diseases on a cellular level in vitro

Investigations of motoneuron diseases on a cellular and molecular level require long-term cultivation of primary cells. Here we present a new culture system in which matured motoneurons interact with their physiological partners like interneurons, astroglia and peripheral glia cells. This enables motoneuron-maturation for up to 3 weeks, while motoneurons consistently reached large diameters of their somata of 30-45 microm, occasionally more than 80 microm. Dissociated rat embryonic ventral spinal cord cells were enriched for motoneurons by density gradient centrifugation and seeded on a non-confluent mono-layer of highly enriched neonatal rat Schwann cells. Immunocytochemical visualization of neuron specific betaIII-tubulin in all neurons and of motoneuron specific non-phosphorylated neurofilament H/M, respectively, revealed that after 3 days in vitro >70% of all neurons were motoneurons. After 20 days in vitro, a motoneuron fraction of 12% was maintained. Motoneurons were susceptible to transient transfection with green fluorescent protein cDNA when liposomal transfection and an enhancer substance were combined. Synaptic connections enabled formation of spontaneously active neuronal networks which provide a culture model to study glutamate excitotoxicity and calcium deregulation on a molecular level. Both mechanisms are implied in the pathophysiology of amyotrophic lateral sclerosis, a neurodegenerative motoneuron disorder.